fish’s core code has been ported from C++ to Rust (#9512). This means a large change in dependencies and how to build fish. However, there should be no direct impact on users. Packagers should see the For Distributors section at the end.

Notable backwards-incompatible changes

• As part of a larger binding rework, bind gained a new key notation.
  In most cases the old notation should keep working, but in rare cases you may have to change a bind invocation to use the new notation.
  See below for details.

• ctrl-c now calls a new bind function called clear-commandline. The old behavior, which leaves a “^C” marker, is available as cancel-commandline (#10935)

• random will produce different values from previous versions of fish when used with the same seed, and will work more sensibly with small seed numbers.
  The seed was never guaranteed to give the same result across systems,
  so we do not expect this to have a large impact (#9593).

• Variables in command position that expand to a subcommand keyword are now forbidden to fix a likely user error.
  For example, set editor command emacs; $editor is no longer allowed (#10249).

• functions --handlers will now list handlers in a different order.
  Now it is definition order, first to last, where before it was last to first.
  This was never specifically defined, and we recommend not relying on a specific order (#9944).

• The qmark-noglob feature, introduced in fish 3.0, is enabled by default. That means ? will no longer act as a single-character glob.
  You can, for the time being, turn it back on by adding no-qmark-noglob to fish_features and restarting fish:

  set -Ua fish_features no-qmark-noglob
  

  The flag will eventually be made read-only, making it impossible to turn off.

• Terminals that fail to ignore unrecognized OSC or CSI sequences may display garbage. We know cool-retro-term and emacs’ ansi-term are affected, but most mainstream terminals are not.

• fish no longer searches directories from the Windows system/user $PATH environment variable for Linux executables. To execute Linux binaries by name (i.e. not with a relative or absolute path) from a Windows folder, make sure the /mnt/c/... path is explicitly added to $fish_user_paths and not just automatically appended to $PATH by wsl.exe (#10506).

• Under Microsoft Windows Subsystem for Linux 1 (not WSL 2), backgrounded jobs that have not been disowned and do not terminate on their own after a SIGHUP + SIGCONT sequence will be explicitly killed by fish on exit (after the usual prompt to close or disown them) to work around a WSL 1 deficiency that sees backgrounded processes that run into SIGTTOU remain in a suspended state indefinitely (#5263). The workaround is to explicitly disown processes you wish to outlive the shell session.

Notable improvements and fixes

• fish now requests XTerm’s modifyOtherKeys keyboard encoding and kitty keyboard protocol’s progressive enhancements (#10359).
  Depending on terminal support, this allows to binding more key combinations, including arbitrary combinations of modifiers ctrl, alt and shift, and distinguishing (for example) ctrl-i from tab.

  Additionally, bind now supports a human-readable syntax in addition to byte sequences.
  This includes modifier names, and names for keys like enter and backspace.
  For example

  • bind up 'do something' binds the up-arrow key instead of a two-key sequence (“u” and then “p”)
  • bind ctrl-x,alt-c 'do something' binds a sequence of two keys.

  Any key argument that starts with an ASCII control character (like e or cX) or is up to 3 characters long, not a named key, and does not contain , or - will be interpreted in the old syntax to keep compatibility for the majority of bindings.

  Keyboard protocols can be turned off by disabling the “keyboard-protocols” feature flag:

  set -Ua fish_features no-keyboard-protocols
  

  This is a temporary measure to work around buggy terminals (#11056), which appear to be relatively rare.
  Use this if something like “=0” or “=5u” appears in your commandline mysteriously.

• fish can now be built as a self-installing binary (#10367). That means it can be easily built on one system and copied to another, where it can extract supporting files.
  To do this, run:

  cargo install --path . # in a clone of the fish repository
  # or `cargo build --release` and copy target/release/fish{,_indent,_key_reader} wherever you want
  

  The first time it runs interactively, it will extract all the data files to ~/.local/share/fish/install/. A specific path can be used for the data files with fish --install=PATH To uninstall, remove the fish binaries and that directory.

  This build system is experimental; the main build system, using cmake, remains the recommended approach for packaging and installation to a prefix.

• A new function fish_should_add_to_history can be overridden to decide whether a command should be added to the history (#10302).

• Bindings can now mix special input functions and shell commands, so bind ctrl-g expand-abbr "commandline -i n" works as expected (#8186).

• Special input functions run from bindings via commandline -f are now applied immediately, instead of after the currently executing binding (#3031, #10126).
  For example, commandline -i foo; commandline | grep foo succeeds now.

• Undo history is no longer truncated after every command, but kept for the lifetime of the shell process.

• The ctrl-r history search now uses glob syntax (#10131).

• The ctrl-r history search now operates only on the line or command substitution at cursor, making it easier to combine commands from history (#9751).

• Abbreviations can now be restricted to specific commands. For instance:

  abbr --add --command git back 'reset --hard HEAD^'
  

  will expand “back” to reset --hard HEAD^, but only when the command is git (#9411).

Deprecations and removed features

• commandline --tokenize (short option -o) has been deprecated in favor of commandline --tokens-expanded (short option -x) which expands variables and other shell syntax, removing the need to use eval in completion scripts (#10212).

• Two new feature flags:

  • remove-percent-self (see status features) disables PID expansion of %self, which has been supplanted by $fish_pid (#10262).
  • test-require-arg disables test’s one-argument mode. That means test -n without an additional argument will return false, test -z will keep returning true. Any other option without an argument, anything that is not an option and no argument will be an error. This also goes for [, test’s alternate name.
    This is a frequent source of confusion and so we are breaking with POSIX explicitly in this regard.
    In addition to the feature flag, there is a debug category “deprecated-test”. Running fish with fish -d deprecated-test will show warnings whenever a test invocation that would change is used. (#10365).

  These can be enabled with:

  set -Ua fish_features remove-percent-self test-require-arg
  

  We intend to enable them by default in future, and after that eventually make them read-only.

• Specifying key names as terminfo names (using the bind -k syntax) is deprecated and may be removed in a future version.

• When a terminal pastes text into fish using bracketed paste, fish used to switch to a special paste bind mode.
  This bind mode has been removed. The behavior on paste is no longer configurable.

• When an interactive fish is stopped or terminated by a signal that cannot be caught (SIGSTOP or SIGKILL), it may leave the terminal in a state where keypresses with modifiers are sent as CSI u sequences, instead of traditional control characters or escape sequences that are recognized by Readline and compatible programs, such as bash and python.
  If this happens, you can use the reset command from ncurses to restore the terminal state.

• fish_key_reader --verbose no longer shows timing information.

• Terminal information is no longer read from hashed terminfo databases, or termcap databases (#10269). The vast majority of systems use a non-hashed terminfo database, which is still supported.

• source returns an error if used without a filename or pipe/redirection (#10774).

Scripting improvements

• for loops will no longer remember local variables from the previous iteration (#10525).
• A new history append subcommand appends a command to the history, without executing it (#4506).
• A new redirection: will try opening th